The development of a secondary battery is among the main concerns of modern industry and society due to the fast development of electronic industry, computer architecture and telecommunication. Especially, electronic goods are becoming smaller and more portable so that a secondary battery which is a part thereof also requires to be thinner and smaller. The most interested battery is a lithium polymer secondary battery which can remove the disadvantages of a lithium ion battery using a liquid electrolyte. The lithium polymer secondary battery has advantages in safety, low cost for preparation, manufacture of a large scale of battery, no limitations to the size or shape of battery, etc. Accordingly, a solid polymer electrolyte for a lithium polymer secondary battery has been researched and developed. Generally, the solid polymer electrolyte has excellent characteristics of a low electron conductivity, a low self-discharge, a high admissibility between electrode and electrolyte, and a coating ability of a large surface area.
Recently, most of researches are related to a plastized polymer electrolyte which is prepared by adding a liquid electrolyte to a polymer matrix. Polymers used for the polymer matrix are poly(ethylene oxide), polyacrylonitrile, poly(vinylidene fluoride), poly(vinyl chloride), etc.
European Patent Publication No. 0279554A2 discloses a polymer electrolyte comprising a complex of a solid polymer and an alkali metal salt, which polymer is capable of forming donor acceptor type bonds with alkali metal ions and is capable of conducting alkali metal ions, characterized in that the complex is associated with a mixture of more than one substituted or unsubstituted 1,3-dioxolane-2-ones. In the patent, poly(ethylene oxide) or poly(propylene oxide) is used as a solid polymer. However, as the glass transition temperature of poly(ethylene oxide) used as a solid polymer is low, the polymer electrolyte film is sticky and therefore it is difficult to handle the film.
U.S. Pat. No. 5,219,679 discloses solid electrolytes composed of solutions of alkali metal salts in a plurality of mixible aprotic organic liquids immobilized in an organic solid polymer matrix which itself can complex with said alkali metal salts, wherein said liquids are ethylene carbonate and propylene carbonate and said polymer is selected from the group consisting of polyacrylonitrile and poly-N-vinyl-2-pyrolidinone. The polymer electrolyte shows a high ionic conductivity of the order of 10.sup.-3 S/cm. However, as polyacrylonitrile is insoluble in an organic solvent, the solid polymer electrolyte should be prepared at a high temperature over 10.degree. C. Also, the polymer matrix has a poor compatibility with the liquid electrolyte so as to occur a phase separation phenomenon which will cause leak out the organic solvent, so called "exudation". The exudation of an organic solvent will result in a serious problem of safety.
Japanese Patent Laid open No. 07-331019 discloses a polymer electrolyte comprising a polymer matrix containing polyacrylonitrile, polybutadiene or polystyrene, such as styrene butadiene rubber polymer, acrylonitrile-butadiene copolymer and acrylonitrile styrene copolymer; a lithium salt such as lithium perchlorate; and an aprotic solvent such as ethylene carbonate and diethyl carbonate. The polymer electrolyte has a high ionic conductivity over 4.0.times.10.sup.-3 S/cm at 25.degree. C., when the polymer electrolyte contains 90% or more of the liquid electrolyte. The polymer matrix has a poor compatibility with the aprotic solvent so as to occur a phase separation phenomenon. Addition of a liquid electrolyte to the polymer matrix in order to obtain a high conductivity over 10.sup.-3 S/cm will cause a poor physical property not so as to prepare an electrolyte film having 100 .mu.m or less.
Accordingly, the present inventors have developed a solid polymer electrolyte having excellent physical properties for preparing a thin electrolyte film and an excellent ionic conductivity, which comprises a polymer complex of poly(acrylonitrile-methyl methacrylate), an aprotic solvent and a lithium salt.
Also, the present inventors have developed a polymer matrix whose polymer is poly(acrylonitrile methyl methacrylate oligooxyethylene ethylether methacrylate), or poly(acrylonitrile methylmethacrylate styrene).